A typical EDM apparatus 1 of the type described above is illustrated in FIG. 1 and includes an electrode 10 supported by a shaft 12 and suspended over a tank 14. Shaft 12 is connected at its opposite end to a piston 16, which is positioned by hydraulic fluid provided by a motor 20 via an electrically controlled servo valve 18. Electrode 10 opposes a workpiece 22 across a machining gap G. Tank 14 is filled with a machining solution 24, whose level in the tank insures that machining gap G is always filled with machining solution 24.
Electrode 10 and workpiece 22 are serially connected by a pair of leads to output terminals of machining power source 30, which includes a DC power supply 32 with a rated output of E volts (V), a switch 34 for switching the power source 30 ON and OFF, an oscillator 36 for controlling the operation of switch 34 and a current limiting resistor 38 with a resistance value of R. Power source 30 supplies an interelectrode voltage V.sub.G to the pair of leads so that a switching current (machining current) I is generated between electrode 10 and workpiece 22.
The current I is represented by the expression I=(E-V.sub.G)/R, where V.sub.G is in the range of about 20 to 30 V during an arc discharge period, 0 V during a short circuiting period and E V during periods when no arc discharge occurs. If the interelectrode voltage V.sub.G is detected and averaged by a smoothing circuit 40, the machining gap G can be controlled in response to the averaged value of the interelectrode voltage V.sub.G. More specifically, when the machining gap G is wide, discharge across the machining gap does not occur and the average voltage, hereinafter denoted V.sub.ave, becomes high, i.e., approaches E. When the gap is narrow, a short circuit between the electrode 10 and workpiece 22 can occur, which results in a reduction in the average voltage V.sub.ave. Accordingly, when the value of V.sub.ave is compared with a reference voltage V.sub.REF, the magnitude and polarity of the difference between these two voltages can be applied to servo valve 18 via an amplifier 42 to properly position electrode 10 with respect to workpiece 22. Thus, the difference between V.sub.ave and V.sub.REF, e.g., control voltage V.sub.C, can be used to control the machining gap G at a substantially constant value.
Electrodes formed from either graphite or metallic materials are used in conventional EDM apparatus 1. Metallic materials such as copper, while low in cost, have low melting points and are thus subject to rapid electrode erosion. Graphite is the electrode material of choice for many applications due to its relatively low cost with respect to more exotic metallic materials such as tungsten. Graphite is also superior in workability to tungsten electrodes, thus allowing complex electrodes to be machined at a relatively low cost.
Experiments using silicon as the electrode material have been conducted. In particular, silicon electrodes having a relatively high resistance have been used to produce workpiece surface finishes with a relatively constant roughness per square irrespective of the surface area of the electrode. As shown in FIG. 2, workpiece surface roughness when using a copper electrode increases proportionately to the surface area of the electrode. One explanation of this observed effect is that the Si electrodes provide a plurality of discharge sites at the surface of the electrode. When a plurality of equipotential discharge points are available, identical electric field gradients are applied at each discharge site and localized discharge occurs at each point. In other words, the surface charge is distributed over a large number of discharge sites with respect to the surface area of the Si electrode.
While Si electrodes provide superior surface finishes, it is difficult to produce usable Si electrodes in three dimensional shapes. Experiments have been conducted using Si powder dispersed in the machining solution in an attempt to achieve similar surface finishing results using conventional electrodes. The experimental results indicate an improvement over conventional electrodes, as indicated in FIG. 2. It will be noted, however, that it is difficult to maintain a uniform Si particle distribution in the machining gap G, particularly in the low flow laminar layers on either side of this gap. Practical devices employing machining solutions containing Si powder require additional or more complex components for maintaining the Si powder in suspension at a proper concentration and for increasing the machining solution 24 flow rates through the machining gap G to keep the Si powder from accumulating in low flow regions of the machining gap G.